Many industries use furnaces for heating and firing materials as part of manufacturing processes. Often these processes must be carried out at high temperatures, i.e. over 1300.degree. F. In many cases high temperature heating processes that use fuel oil or gas directly as heat sources, are very low in thermal efficiency since most of the energy in the fuel goes to heat air and gas which in turn maintains the temperature of the furnace walls and heats the material being processed. The furnace gas composed of the products of combustion and excess air is often exhausted while still at temperatures approximating that of the furnace walls, thus wasting substantial amounts of heat energy.
In the metal forging industry, oil and gas fired furnaces are employed to heat the work in process to forging temperatures. The work pieces are placed in the heated furnace zone through a narrow slot running essentially the full length of the furnace.
In such slot furnaces used for forging, it is known that the processed materials absorb a maximum of 15% of the heat generated and often as low as 2% of the heat generated in the combustion zone. Typically about 10% of the heat is lost through the furnace walls and the remaining 75-88% of the energy is exhausted from the furnace in the hot gas exhaust and lost to the environment.